1. Field of the Invention
This invention relates to an improved bandgap reference voltage generator and, more particularly, relates to a bandgap reference voltage generator whose operation is compensated to produce an output reference voltage that is independent of variations in supply voltage, V.sub.CC.
2. Discussion of Background and Prior Art
Emitter-coupled logic (ECL) is a widely utilized logic family for high performance products. ECL has the shortest propagation delay of any logic form. With ECL logic, superior comparator functions and high-speed analog-to-digital conversion may be accomplished. ECL logic is utilized in such diverse applications as instrumentation, computers, phase-array radar, telecommunication systems, and a host of modern electronics applications where high performance is required or desired. It is important to preserve this high performance potential when ECL circuits are designed and fabricated.
To ensure that integrated circuits embodying ECL logic achieve maximum performance, a bandgap reference voltage is commonly generated on-chip and is used to control the base of the main current source transistor that establishes the magnitude of the current that flows either through a reference transistor or that flows both through a reference transistor and through input transistors. The bandgap reference voltage, designated V.sub.REF or V.sub.CS, has the characteristic that it is stable and that it tracks variations in processing and changes in operating parameters such as temperature. See, e.g., Integrated Circuits Applications Handbook, ed. A. H. Seidman pp. 498-499 (1983). See also D. A. Hodges, et al, Analysis and Design of Digital Integrated Circuits, pp. 271-283 (McGraw-Hill 1983). In ECL circuits a reference voltage V.sub.BB is also generated from V.sub.CS and supplied to the gate of the reference transistor in order to establish the threshold level for the recognition of a high digital logic state.
The supply voltage, V.sub.CC, is generated externally and introduced to a packaged circuit through a dedicated pin. For ECL circuits an externally supplied V.sub.CC is specified as being acceptable if it lies within a range from about 4.5 volts to about 5.5 volts. Thus, an external power supply may provide a voltage anywhere within this range and the integrated circuit will function properly. Since the supply voltage V.sub.CC is used to provide power to the internal bandgap reference voltage generator as well as to all other circuit elements, this generator must be able to operate properly over values for V.sub.CC within this range. In practice it has been found that the operation of conventional bandgap voltage generators is dependent upon the static value for V.sub.CC, i.e., upon the baseline value for V.sub.CC without taking transients into account. Thus, in a DC or static sense V.sub.CS is dependent on V.sub.CC. If V.sub.CS varies, the total chip current, I.sub.CC, will vary, the logic swing will vary and the main current source transistor may saturate (transistor 60 in FIG. 3). If I.sub.CC varies then the design of the integrated circuit is made more difficult and its operation less reliable. If the logic swing is too high then the input transistor on the ECL differential pair may saturate; if the logic swing is too low then noise margins are reduced. It would be highly desirable to provide a bandgap reference voltage generator whose V.sub.CS output is independent of the static value for V.sub.CC over the allowable range for V.sub.CC. In addition, even for a supply voltage V.sub.CC having a value precisely in the center of the allowable voltage range or for a supply voltage which stays precisely at any particular allowable value there will be transients in the supply voltage V.sub.CC due to instabilities in the power supply and to transient currents induced by switching on the output of associated logic gates. These transients will typically penetrate through the bandgap reference voltage generator and alter the instantaneous value for V.sub.CS. Thus, in an AC or transient sense V.sub.CS is dependent on V.sub.CC. These variations are highly undesirable on large integrated circuits as they are likely to occur unevenly across the chip thereby producing perturbations in overall circuit performance. It would be desirable to make the instantaneous value for V.sub.CS immune to such transients. And, V.sub.CS will vary over temperature, an undesirable feature for a supposedly stable reference voltage generator. It would also be desirable to generate a bandgap reference voltage V.sub.CS with no temperature dependence.
It is therefore an object of the present invention to provide a bandgap reference voltage generator which includes compensation circuitry to thereby produce a stable reference voltage, V.sub.REF, over the allowable range of operation for supply voltage, V.sub.CC.
It is another object of the present invention to provide a bandgap reference voltage generator in which transients from V.sub.CC do not couple through the bandgap reference voltage generator to V.sub.CS.
And it is an additional object of the present invention to provide a bandgap reference voltage generator for which V.sub.CS is not dependent on temperature.